You're right that the Sneaky Feeder isn't as sneaky as she could be. The problem was that she had to fit into the same feeder system as the other three. Creating that system took me quite a while, so I didn't have time to create a more complex version that supports more secrecy, but that might be something I do in the future!
I want to add some interactions between the girls in the future, too, instead of them all being completely separate. Your idea about a girl trying to get another to break up with you could work well there.
I'm still at an early stage of planning how the girls will be converted into feedees. It'll probably involve special feedist interactions the player can do with a girl, such as complimenting her gain, convincing her that she's a real glutton, showing physical affection for her pudgy parts (like belly rubs), feedist talk during sex... all culminating in revealing to her that you're a feeder and intentionally trying to fatten her up. All of these actions could have a chance to succeed or fail. If you're successful enough, she'll eventually become a feedee and begin gaining intentionally.